1. Field of the Invention
This invention relates to the field of data communications devices, and more particularly to spread spectrum data recovery devices.
2. Previous Art
Spread spectrum systems have been known at least since the disclosure of U.S. Pat. No. 3,160,711 to Schroeder in 1964. Pat. '711 relates to a communication apparatus for transmitting a message wave. The transmission apparatus creates pulse groups for representing the information in the message, and which have two characteristics. One characteristic of the pulse group indicates the amplitude of a time spaced sample of the message wave, and the other characteristic of the pulse group indicates the intended receiver of the pulse group. The transmission apparatus further has means for widening each pulse of the group before transmitting the group. In parlance adopted since Schroeder's disclosure, `widening` is called `spreading`, and it is accomplished by replacing the presence or absence of one pulse in each time period of the pulse group with a group of pulses arranged in a sequence called a `spreading code.`
Data communication systems which use spread spectrum have historically been designed to fulfill requirements for communications performance and reliability that are not achievable with prior improvements in communications technology such as frequency modulation (FM) and single-side band (SSB) technologies. Such requirements include immunity from strong interfering signals, receiving signals that are very weak, providing many communication channels within a restricted band of frequencies, scrambling the transmitted information so that it cannot be received by unwanted receivers, and processing the transmitted signal so that the fact that it is even being sent is concealed.
These requirements historically have arisen in applications such as military and aerospace communications systems. In military applications, interfering signals can be caused by the intentional efforts of an enemy. Additionally in military applications, avoidance of detection is an asset. In aerospace applications, difficulties arise due to the weakness of signals received over great distances. Cost has seldom, if ever, been a design constraint in any of the above systems.
Characteristic of spread spectrum communication systems are improved security, greater immunity from interference, and more efficient use of spectrum space, compared with the characteristics of communications systems which do not use spread spectrum technology. These characteristics are attractive for certain consumer and industrial applications. However, the cost of implementation has been an obstacle to adoption of spread spectrum technology in the commercial sector.
One element of prior art spread spectrum receivers that significantly contributes to the expense of spread spectrum equipment is the phase-locked loop which synchronizes the receiver's demodulation clock with the modulation clock of the transmitter whose signal is being received. The expense of this element arises not only from the cost of its individual components, but also from the labor cost to adjust it for proper operation during manufacture.